There are a number of known techniques for determining the position of an apparatus using radio frequency signals. Some popular techniques relate to use of the Global Positioning System (GPS), in which multiple satellites orbiting Earth transmit radio frequency signals that enable a GPS receiver to determine its position. However, GPS is often not very effective in determining an accurate position indoors.
Systems have been proposed for allowing location determination indoors using multi-antenna arrays and a single receiver. Here, different elements of the array are connected to the receiver in turn by a switch. Samples of the signals thus provided are processed to determine a bearing from which the signal was received.
In order to obtain accurate measurements in such systems, the receiver is configured to determine and compensate a difference in frequency between the RF carrier of the received signal and a downconversion frequency used at the receiver. Because of the switching taking place at RF, many conventional techniques for calculating carrier frequency offset are not applicable. There are, however, various ways in which carrier frequency offset can be calculated. One technique, although in a slightly different type of system in which a multi-element antenna is used for transmission to a single-element receiver, is disclosed in WO 2009/56150.